1. Field of the Invention
The invention is related to curing agents for epoxy resin systems that provide excellent mechanical properties and impart toughness to the cured resin system. The aromatic amine curing agents according to the invention have utility in commercial aircraft, aerospace structures, or other applications requiring high performance epoxy resins. This aromatic amine, used alone or in combination with other amines, curing agents, or compatible modifiers, may be useful as a curing agent for epoxy resins for resin transfer molding (RTM) resins, vacuum assisted resin transfer molding (VARTM) resins, pultrusion resins, film adhesives, prepreg resins, and other composite manufacturing type applications.
2. Description of the Related Art
Fiber reinforced polymeric matrix composites are being used and targeted for many structural applications in areas such as aircraft, aerospace, automotive, and sporting goods. Typically, these high performance composites contain orientated continuous carbon fibers, woven fabric or unidirectional, cured in a thermoset matrix. Glass or Kevlar® fibers may also be used in these applications. While there are many types of thermoset matrices that find use in composite applications, epoxy resins dominate the market due to ease of use, excellent properties, and relatively low cost.
Epoxy resins are compounds that contain an oxygen atom connected to two adjacent carbon atoms. A variety of epoxy resins are known and commercially available.
Epoxy resins of the type of diglycidyl ethers of bisphenol A are most easily available under trade names of Epon 828, Epon 1001, Epon 1002, Epon 1004, Epon 1007 from Shell Chemical Co. Their chemical structure corresponds to the formula: 
wherein n is the degree of the polymerization.
Novolac type epoxy resins are obtained by reacting novolacs, which are reaction products of phenol and formaldehyde, with epichlorohydrin and correspond to the general formula: 
wherein R is 
and n is the degree of polymerization. Cresol may be substituted in place of phenol to form corresponding novolacs.
Another epoxy resin that is extensively used in the aerospace industry in primary and secondary structures is tetraglycidyl methylene dianiline (TGMDA). TGMDA has the following chemical structure: 
Thermoset matrix prepregs are the most widely used materials for manufacturing high performance composites. These pre-engineered laminating materials have a discrete resin/fiber ratio that require further lay-up of the continuous fiber plies to produce the final composite. The prepreg layup is then usually placed in an autoclave or press and cured under elevated temperature and pressure. While prepreg materials may be the most common method for producing advanced composites at present, lower cost manufacturing methods, such as resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM) are being readily adopted for replacing this older technology. These methods for developing high performance composites consist of infusing a liquid resin system into a dry fabric reinforcement or preform of fibers, followed by curing at appropriate temperature and pressure. Different from prepreg resins, RTM or VARTM resins must be low viscosity during infusion, having an adequate out-time, and be homogeneous before cure. This limits the use of some typical prepreg curing agents and some resins since they may impart high viscosity or can be solids in particulate form. In addition, conventional toughness modifiers, such as thermoplastics and elastomers, are very limited to usage in these systems since they increase viscosity to an unacceptable level.
One method of making composite materials is by resin transfer molding (RTM). This is a process by which a resin system is transferred while at relatively low viscosity and under pressure into a closed mold with all of the important reinforcements and inserts already in place. The resin system can be prepared by premixing and placing the resin system into a resin injection pot or by metering components from separate pots at the appropriate mix ratio to an in-line static mixer or mixing zone. The resin system is then injected into the mold which is maintained under low pressure or under vacuum. The mold is often filled with resin while under vacuum to eliminate air from the mold space, to assist in resin injection and to aid in the removal of volatiles. The viscosity of the resin system dictates whether pot and/or mold heat is required. Low resin viscosity at the injection temperature is desirable to obtain best mold filling and mold wetting. After the mold is filled, it is sealed and heated in accordance with the appropriate cure schedule. The resulting molded part can then be removed from the mold and post-cured as necessary.
In order to achieve good fiber impregnation and low void content during RTM processing, resin viscosity below about 2000 cps at the injection temperature is highly desired, with resin viscosity below 1000 cps being preferred, and below 100 cps, most preferred. Further, the resin system must maintain this low viscosity for a period of time sufficient to completely fill the mold and impregnate the fiber preform. For RTM processing, such time is frequently measured in terms of the pot life of the resin, which can be defined as the time required for the resin to double its viscosity value. A resin pot life of at least 1 hour, and preferably two hours or more, is generally required for production of parts via RTM.
Vacuum assisted resin transfer molding (VARTM) is a composite manufacturing method by which the resin is impregnated into the continuous fibers or fabric through an atmospheric pressure differential between the resin environment and the composite fibrous reinforcement environment. Usually, the fiber reinforcement is placed under an air impermeable bag, made of Nylon film or silicone sheeting that is on top of aluminum or steel plate or “tool”. Air is usually evacuated from space between this flexible bag and tool using a vacuum pump at one end of the part to be made. Opposite this end, a resin distribution line is usually connected to a valve that can be opened and allows the resin to be pushed by the atmosphere into the evacuated bag throughout the fibrous reinforcement. Usually, a resin distribution medium, or a more porous medium than the fibers is placed on top of the fibrous reinforcement for faster impregnation. Many different types of VARTM manufacturing methods exist and methods for resins distribution are described further in U.S. Pat. Nos. 4,902,215 and 5,052,906, which are hereby incorporated by way of reference in their entirety.
Modified or unmodified resin systems can be used in conjunction with woven fabric or non-woven mat reinforcements or preforms to directly produce advanced composite parts via processes such as RTM, RFI, VARTM. In these processes, the resin and fiber are combined during the actual part molding process. Any of the fiber types known to one skilled in the art may be utilized, with the most preferred type being determined by the performance characteristics of the application.
Epoxy based resin systems used for RTM and VARTM applications may consist of either one or two component systems. Single component resin systems do commercially exist, but usually require cold storage similar to prepreg materials since they contain the curing agent and or catalysts in the resin. A RTM resin manufactured for advanced composites that is representative of this type of technology is RTM 6, by Hexcel. Two part resin systems differ in that the epoxy resin and curing agent or catalyst, are stored in separate containers and therefore can be stored at ambient temperature without advancing the resin or promoting reaction. Once both parts are mixed, the composition can be used for infusion, similar to that of a single component resin system.
Resin systems containing an epoxide resin and aromatic amine hardener are often used in advanced composites since they possess a balance of properties generally required for such high performance applications. An example of a resin systems that has been used extensively in aerospace primary and secondary structures is based on tetraglycidyl-methylenedianiline [TGMDA] epoxy resin and 4,4′-diaminodiphenylsulfone [4,4′-DDS] curing agent. Fiber reinforced composites based on this resin system have excellent compression properties and high thermal performance. One drawback, from a mechanical property standpoint, of this type of composite system is its inherent brittle nature. As such, thermoplastic and rubber materials have been added for increasing toughness of such systems. While this type of resin system makes excellent prepreg materials, problems arise when using DDS for liquid molding systems. In its unmodified form, DDS is a powder that requires melting or pre-adducting into the epoxy resin before curing so that the two components are homogeneous and single phase. This generally results in a very high mixed viscosity, and renders these formulations only useful for manufacturing techniques such as resin film infusion. An example of this technology is 3501-6 or 3501-RC manufactured by Hexcel, which has been used for resin film infusion for decades. Complicating the use of DDS for two part resins is that it is relatively insoluble in other aromatic amine-type curing agents. Therefore, the use of this curing agent for two part, epoxy/aromatic amine cured systems is limited to a small percent.
Presently, the most widely used two component RTM and VARTM resin system is one manufactured by Resolution Performance Products, consisting of Epon 862 and curing agent W, a diglycidyl ether of bisphenol F epoxy and an liquid aromatic amine diethylene toluene diamine (DETDA), respectively. Unique to this composition is the liquid aromatic amine curing agent, which is semi-latent and provides moderate thermal properties. This system, although excellent for processing, suffers from a low balance of mechanical properties, not approaching high performance matrix resins used for aerospace grade prepreg systems. The desire to manufacture high performance two part VARTM and RTM resins has driven research and development to provide higher performing matrix resins coupled with desired low cost processing. At present, only a few commercial liquid aromatic amine curing agents exist, which are typically based on DETDA. This product, although low in viscosity, has not provided the necessary mechanical properties for use as a matrix in high performance composites. Liquid mixtures of aromatic amines have previously been made and sold in production quantities. The most common is known as Tonox 60/40 which consists of 60 percent m-phenylenediamine and 40 percent methylenedianiline, manufactured by Uniroyal. This combination and all previous known liquid aromatic amines do not pass the required combination of long infusion and processing time, high mechanical properties, adequate toughness, and non-carcinogenicity.
Presently, no individual amine structure or combination or blend satisfies the combination of low viscosity processing, low temperature gelation/vitrification and high mechanical properties required for structural aircraft composites.